Such an arrangement is shown, for example, in U.S. Pat. No. 5,675,401.
Lasers are primarily used as light sources for microlithography because they emit a very narrow band of light and, in the case of excimer lasers, light of very low wavelength in the deep ultraviolet range is emitted. The time-dependent and spatial coherence of these lasers as well as the small cross section and low divergence of the light beam are, however, not adapted to the situation for illuminating devices for microlithography.
Cross section and divergence cannot be changed independently by singular imaging optical elements in the cross section of the light beam as the light-conductance value cannot be increased. The light-conductance value is defined as the luminous flux divided by the luminance. In this context, reference can be made to the text of M. Young entitled "Optics and Lasers", Springer Verlag (1984), page 51 and the text of K. Mutze et al entitled "ABC der Optik" Verlag Dausien (1961), starting at page 477. The Lagrange invariant is closely related to the maintenance of the light-conductance value. In this connection, reference may be made to the text of M. Young cited above at pages 50 and 51.
Scattering elements are known to increase the light-conductance value. Frosted glass plates or quartz glass plates having statistically orientated microfaces, which act to refract, reflect or diffract, are used for this purpose. The scatter profile of such scattering plates is very intense in the center but also still distributes considerable energy at large angles in a tail portion of a distribution curve of the energy.
A targeted distribution of rays having divergence magnification and cross section magnification is obtainable with lens rasters which are available for the ultraviolet range and the DUV range (deep ultraviolet range).
Diffractive optical raster elements in quartz can be produced by photolithography in the most different embodiments and can be substituted for the raster lens plates.
U.S. Pat. No. 4,936,665 discloses a wafer illuminating system having a wafer stepper, projection objective, excimer laser, radiation forming optics and expansion optics and an illuminating system having several lens groups which, in turn, has an entry pupil. Two divergence generating elements are provided in the entry pupil and in a further plane. In contrast to the present invention, the second plane is likewise a pupillary plane and the two elements are stochastic scattering plates having scattering silicon crystallites. An oscillating mirror is additionally provided.
The foregoing functions to reduce the spatial coherence. The significance of the adaptation of the spatial coherence by magnification of the effective light source for wafer illuminating systems is described.
The two scattering plates cannot possess an anamorphotic effect. Zoom lens, axicon lens and glass rod are not described. A nonrastered diffractive optical element in the form of a blazed transmission grating having concentric multiplateau rings is provided in the pupil of the projection objective to correct aberrations of the wavefront.